Imaging Observing Strategies

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The gaps between the three detectors in GMOS-S cause gaps in the imaging field about 4.9 arcsec wide. For GMOS-N, the inter-chip gaps between the three Hamamatsu detectors are 0.5 arcsec wider (5.4 arcsec). If continuous coverage of your field is needed, you will need to obtain multiple exposures and dither between the exposures. Due to additional bright columns at either side of the chip gaps, a minimum dither step of 10 arcsec in the X-direction (p-direction on the OT) is recommended.

(The gap width for the old GMOS-N e2v DD devices was 2.8 arcsec, and the recommended minimum dither step in X-direction 5 arcsec.)

For unbinned observations the pixel scale is 0.0807 arcsec/pixel (GMOS-N Hamamatsu) and 0.080 arcsec/pixel (GMOS-S Hamamatsu). (The pixel scale of the old GMOS-N e2v DD devices was 0.0728 arcsec/pixel.) If you are requesting imaging in image quality 70-percentile or worse, consider binning
the CCDs 2x2 giving an effective pixel scale of 0.161 arcsec/pixel (GMOS-N) and 0.160 arcsec/pixel (GMOS-S). This cuts the
overheads from the readout from ~83 sec to ~24 sec per frame(*).

The original GMOS EEV CCDs had strong fringing in the z' and i'-filters. The peak-to-peak amplitude was about 5% for the z'-filter. The new GMOS-N and GMOS-S Hamamatsu CCDs have less fringing in both the i'
and z' filters. The peak-to-peak amplitude is about 2.5% for the Y-filter. For sparsely populated fields, fringe frames can be constructed from the science
images. For crowded fields or extended objects, you may consider obtaining
additional sky observations for the construction of fringe frames. Example fringe frames are available (GN, GS).
To construct a reasonably good fringe frame you will need a minimum of six dithered
science exposures.

Are the Baseline Calibrations sufficient for your
program? If you need photometry to
better than 5%, you will need to add standard stars to your observations to determine
extinction.

(*) Final characterization of the read-out overheads for the GMOS-N Hamamatsu detectors is ongoing.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five participant countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Brazilian Ministério da Ciência, the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, Tecnologia e Inovação and the Korea Astronomy and Space Institute (KASI). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.